Method and apparatus for generation and amplification of light in a semi-guiding high aspect ratio core fiber

Abstract

A planar laser gain medium and laser system. The novel laser gain medium includes an active core having a high aspect ratio cross-section with a fast-axis dimension and a slow-axis dimension, signal claddings adapted to form reflective boundaries at fast-axis boundaries of the core, and a material adapted to minimize reflections at slow-axis boundaries of the core. In an illustrative embodiment, the laser gain medium is an optical fiber. The core and claddings form a waveguide adapted to control modes propagating in the fast-axis direction. When the laser gain medium is employed as a laser oscillator, a high reflectivity mirror and an outcoupler are positioned at opposite ends of the core to form a laser resonator adapted to control modes in the slow-axis direction.

1 . A laser system comprising:
 a laser gain medium having an active core with a high aspect ratio cross-section;   pumping means for exciting said laser gain medium to generate laser energy;   waveguiding means for mode control of said laser energy along a fast-axis direction of said core; and   resonator means for mode control of said laser energy along a slow-axis direction of said core.   
     
     
         2 . The system of  claim 1  wherein said laser gain medium is an optical fiber. 
     
     
         3 . The system of  claim 1  wherein said waveguiding means includes signal claddings disposed in contact with fast-axis boundaries of said core. 
     
     
         4 . The system of  claim 3  wherein said signal claddings have a refractive index less than a refractive index of said core such that light in said core is trapped by total internal reflection in said fast-axis direction. 
     
     
         5 . The system of  claim 4  wherein said refractive index of said signal claddings is reduced relative to that of said core by modifying dopant concentrations in said claddings. 
     
     
         6 . The system of  claim 4  wherein said refractive index of said signal claddings is reduced relative to that of said core by including a micro-structure in said claddings. 
     
     
         7 . The system of  claim 3  wherein said claddings include a micro-structure adapted to trap light in said core according to principles of photonic bandgaps. 
     
     
         8 . The system of  claim 1  wherein a thickness of said core is matched to a numerical aperture of said core such that said core is a single-mode planar waveguide in said fast-axis direction. 
     
     
         9 . The system of  claim 1  wherein a thickness of said core is adapted to support multiple modes in said fast-axis direction. 
     
     
         10 . The system of  claim 9  wherein said system further includes means for stripping-out higher-order modes from said core. 
     
     
         11 . The system of  claim 10  wherein said system includes means for coiling said laser gain medium with a coil radius adapted to present a high loss to higher-order modes but a low loss to a lowest-order mode. 
     
     
         12 . The system of  claim 1  wherein said active core is optically open in said slow-axis direction. 
     
     
         13 . The system of  claim 1  wherein width and gain value of said active core and length of said laser gain medium are specified in such a manner to enable gain guiding in a slow-axis direction. 
     
     
         14 . The system of  claim 1  wherein said resonator means includes a mirror and an outcoupler positioned at opposite ends of said core to form a laser resonator. 
     
     
         15 . The system of  claim 14  wherein sizes, curvatures, and spacing of said mirror and outcoupler are chosen to control modes propagating in said core in said slow-axis direction. 
     
     
         16 . The system of  claim 14  wherein said mirror and outcoupler form a stable resonator. 
     
     
         17 . The system of  claim 16  wherein a size of said outcoupler is chosen to produce a planar resonator Fresnel number less than unity. 
     
     
         18 . The system of  claim 16  wherein a length of said resonator is chosen such that a lowest-order mode overfills said core such that its average size exceeds approximately twice the width of said core. 
     
     
         19 . The system of  claim 18  wherein a size of said outcoupler is chosen such that said average mode size exceeds said outcoupler size by approximately a factor of two. 
     
     
         20 . The system of  claim 14  wherein said mirror and outcoupler form a short unstable resonator. 
     
     
         21 . The system of  claim 1  wherein said system further includes means for applying local and/or distributed twists of said laser gain medium to create and control an effective planar lens. 
     
     
         22 . The system of  claim 2  wherein said resonator means are integrated into said fiber. 
     
     
         23 . The system of  claim 2  wherein said system further includes an end-cap fused to an end of said fiber for expanding an output laser beam. 
     
     
         24 . The system of  claim 23  wherein said end-cap has an exit surface that is shaped to control a divergence of said output beam. 
     
     
         25 . The system of  claim 23  wherein said end-cap has an exit surface that is shaped to collimate said output beam. 
     
     
         26 . A laser system comprising:
 an optical fiber having an active core with a high aspect ratio cross section, and signal claddings positioned in contact with said core and adapted to control modes along a fast-axis direction of said core;   one or more pump apparatuses for coupling pump energy into said core to generate laser energy; and   a mirror and an outcoupler positioned at opposite ends of said fiber core to form a laser resonator adapted to control modes of said laser energy along a slow-axis of said core.   
     
     
         27 . The system of  claim 26  wherein said active core is optically open in said slow-axis direction. 
     
     
         28 . The system of  claim 26  wherein width and gain value of said active core and the length of said optical fiber are specified in such a manner to enable gain guiding in a slow-axis direction. 
     
     
         29 . The system of  claim 26  wherein said mirror and outcoupler are integrated into said fiber. 
     
     
         30 . The system of  claim 26  wherein said fiber further includes a glass foil surrounding said core and signal claddings. 
     
     
         31 . The system of  claim 26  wherein said system further includes a mechanism for applying local and/or distributed twists of the fiber to create and control an effective planar lens in said laser resonator. 
     
     
         32 . The system of  claim 26  wherein each pump apparatus is adapted to focus a pump beam at a slanted angle into said fiber. 
     
     
         33 . The system of  claim 26  wherein said pump apparatuses are positioned near edges of said fiber beyond said core. 
     
     
         34 . The system of  claim 26  wherein said pump apparatuses are oriented facing toward said mirror. 
     
     
         35 . The system of  claim 26  wherein said pump apparatuses are concentrated closer to said outcoupler than said mirror. 
     
     
         36 . The system of  claim 26  wherein said pump apparatuses include optical prisms placed in optical contact with said fiber. 
     
     
         37 . The system of  claim 26  wherein said pump apparatuses include fiber pigtails adapted to couple pump light into edges of said fiber. 
     
     
         38 . The system of  claim 37  wherein said fiber pigtails are integrated into said fiber. 
     
     
         39 . The system of  claim 38  wherein said fiber pigtails have high aspect ratio cores. 
     
     
         40 . The system of  claim 26  wherein said system further includes an end-cap attached to an outcoupler end of said fiber for expanding an output laser beam. 
     
     
         41 . The system of  claim 40  wherein said end-cap has an exit surface that is shaped to control a divergence of said output beam. 
     
     
         42 . The system of  claim 40  wherein said end-cap has an exit surface that is shaped to collimate said output beam. 
     
     
         43 . An optical arrangement comprising:
 a plurality of planar core fibers arranged sequentially end to end, each fiber having a slow axis oriented orthogonal to slow axes of neighboring fibers and   focusing optics adapted to transform a near-field profile from an output of one fiber to a far-field profile at an input of a subsequent next fiber.   
     
     
         44 . The arrangement of  claim 43  wherein each fiber includes a high-aspect ratio cross-section core, and claddings positioned in contact with said core to control modes along a fast axis of said core 
     
     
         45 . The arrangement of  claim 43  wherein each fiber is optically open along a slow axis of said core. 
     
     
         46 . The arrangement of  claim 43  wherein said focusing optics include a positive lens disposed between ends of two neighboring fibers. 
     
     
         47 . The arrangement of  claim 46  wherein ends of neighboring fibers are spaced at a distance equal to twice a focal length of said lens. 
     
     
         48 . The arrangement of  claim 43  wherein said focusing optics include a glass gradient index lens disposed between ends of two neighboring fibers. 
     
     
         49 . The arrangement of  claim 48  wherein ends of neighboring fibers are spaced at a distance equal to a focal length of said lens. 
     
     
         50 . The arrangement of  claim 43  wherein said arrangement further includes a mechanism for applying local and/or distributed twists of said fibers to create and control an effective planar lens. 
     
     
         51 . The arrangement of  claim 43  wherein said arrangement further includes one or more end-caps attached to ends of said fibers for expanding a signal beam. 
     
     
         52 . The arrangement of the  claim 43  wherein said fibers include one or more additional components that are integrated into said fibers. 
     
     
         53 . The arrangement of  claim 43  wherein said arrangement forms a laser amplifier. 
     
     
         54 . The arrangement of  claim 43  wherein said arrangement further includes a mirror and an outcoupler positioned at opposite ends of said arrangement to form a laser resonator. 
     
     
         55 . A laser system comprising:
 an optical fiber having an active core with a high aspect ratio cross section, and signal claddings positioned in contact with said core and adapted to control modes along a fast-axis direction of said core; and   a material disposed in contact with said core at narrow edges of said core and adapted to minimize reflections in a slow-axis direction of said core; and   a plurality of on-ramp fiber pigtails integrated into said fiber for coupling pump energy into said active core to generate laser energy.   
     
     
         56 . The system of  claim 55  wherein said core is optically open in said slow-axis direction. 
     
     
         57 . The system of  claim 55  wherein a material disposed has a refractive index that provides anti-guiding along a slow-axis direction. 
     
     
         58 . The system of  claim 55  wherein width and gain value of said active core and length of said optical fiber are specified in such a manner to enable gain guiding in a slow-axis direction. 
     
     
         59 . The system of  claim 55  wherein said fiber pigtails have high aspect ratio cores. 
     
     
         60 . The system of  claim 55  wherein said fiber pigtails have rectangular cores. 
     
     
         61 . The system of  claim 55  wherein said fiber pigtails have cores shaped to match a beam shape of said pump energy. 
     
     
         62 . The system of  claim 55  wherein said fiber pigtails are adapted to couple pump energy at a slanted angle into said fiber. 
     
     
         63 . The system of  claim 55  wherein said fiber pigtails are adapted to couple pump energy into edges of said fiber. 
     
     
         64 . The system of  claim 55  wherein said system further includes a mirror and an outcoupler positioned at opposite ends of said fiber core to form a laser resonator adapted to control modes along a slow-axis of said core. 
     
     
         65 . The system of  claim 64  wherein said fiber pigtails are oriented facing toward said mirror. 
     
     
         66 . The system of  claim 64  wherein said fiber pigtails are concentrated closer to said outcoupler than said mirror. 
     
     
         67 . The system of  claim 64  wherein said mirror and outcoupler are integrated into said fiber. 
     
     
         68 . The system of  claim 55  wherein said system further includes an end-cap attached to an outcoupler end of said fiber for expanding an output laser beam. 
     
     
         69 . The system of  claim 68  wherein said end-cap has an exit surface that is shaped to control a divergence of said output beam. 
     
     
         70 . The system of  claim 68  wherein said end-cap has an exit surface that is shaped to collimate said output beam. 
     
     
         71 . A device for coupling pump energy into a planar fiber comprising:
 a glass prism placed in optical contact with a surface of said planar fiber and   one or more fiber pigtails fused within said glass prism at a large angle to a prism surface and having an end in contact with said surface of said planar fiber.   
     
     
         72 . The device of  claim 71  wherein a numerical aperture of a core of said fiber pigtail is no more than half of a numerical aperture of a pump guiding channel in said planar fiber. 
     
     
         73 . The device of  claim 71  wherein a tilt angle of said fiber pigtail inside said prism is close to a numerical aperture of a core of said fiber pigtail. 
     
     
         74 . The device of  claim 71  wherein a face of said prism in contact with said planar fiber beyond a core footprint of said fiber pigtail is partially coated with a high reflectivity coating. 
     
     
         75 . A method for generating laser energy, the method comprising:
 arranging a plurality of planar core fibers sequentially end to end, with each fiber having a slow axis oriented orthogonal to slow axes of neighboring fibers; and   transforming a near-field profile from an output of one fiber to a far-field profile at an input of a subsequent next fiber.   
     
     
         76 . A method for coupling pump energy into an optical fiber, the method comprising:
 integrating a plurality of on-ramp fiber pigtails into said fiber;   coupling pump energy from a pump source to said fiber through said pigtails; and   matching cores within said pigtails to a beam shape of said pump energy.